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Truncated Grid References
    Date first written: 2000.01.01     Last Review: 2006.11.17

Table of Contents
1. Review of Full Grid References
2. Grid Lines on Maps
3. Grid Labels in Articles
4. Conversion of truncated grid references
5. UTM Zone Boundaries
6. Using Paper Maps and the Converter

1. Review of Full Grid References
Full Grid References

Before discussing truncated grid references, let us review what a full UTM grid reference looks like. A full UTM grid reference has a 3 digit "zone code", then 6 digits for the "easting", and 7 digits for the "northing". Below is the full UTM code for Mount Robson:

11U 355644 5886712
Now look up Mount Robson in Bivouac, and find this UTM location on the Mount Robson page. Note that every mountain page contains two such UTM grid references. The first one is WGS84 datum and the second is NAD27 datum. (A discussion of Map Datums is a separate discussion, for now, all you have to know is that there is a difference). Most maps in Canada are still the old NAD27 datum.

GPS units always display the FULL grid reference, and never the truncated or "partial" grid reference. Furthermore, they usually "pad" the easting with an extra leading zero. In the number below, note the extra zero in front of the "3".


  11U 0355644 5886712

This makes both the easting and northing the same length. This makes it easy to pick out the truncated grid reference when in the field: it is always digit #3,4, and 5, for both easting and northing. If you read the above number on your GPS, and somebody was beside you with a map, you'd tell them you were at 556-867.

2. Grid Lines on Maps
On 1:50,000 map sheets, the grid lines are labelled using only the 3rd and 4th digits of the full UTM. For example, a given map might have grid lines labelled 64, 65, 66, 67 and so on. Each of these corresponds exactly to 1 km. However usually the first or second grid line in the lower left corner of the map is labelled with the full UTM code. The label is in blue, like the grid lines, with the 2 digit gridline number in larger font. For example, if you look at how the grid lines are labelled for North Vancouver, you'll see:

64 465000m. E.     66 67 68 69 70 71 72

Note that the first digit of 465000 is small print. This is the "leading digit of the easting.". (the most significant digit). The last three digits are the 3 least significant digits.

The same multi-font trick is used for the full labels of the bottom and top horizontal grid lines. We refer to the first TWO digits of the northing as the "leading digits of the northing".

5482000m
  81
  80
  79
  78
  77
  76

3. Grid Labels in Articles
As with the grid line labels, authors sometimes omit the leading digits when quoting grid references. They just specify the mapsheet number such as "83 E/15", and then truncated grid references throughout their article. The diagram below shows a full grid reference, with the part you would write down as a larger font.

NAD27 11 U   355639     5886495.

Note that the truncated grid reference is the 2nd,3rd and 4 digits of the first number (easting), and the 3rd, 4th and 5th of the second number (northing).

4. Conversion of truncated grid references
To convert back to a full UTM grid reference, any program or GPS device has to know what the leading digits are. It also has to know the zone. One way to get them would be to look at the paper map. Usually, unless the map crosses a UTM boundary, any grid reference from that mapsheet will have the same leading digits.

A better way to convert a grid reference is just to look up a nearby mountain page in Bivouac, and copy down the leading digits. (they are the small numbers that preceed the larger numbers in the UTM reference. These digits can then be supplied to a conversion program or GPS to restore the full UTM code. For convenience, the Bivouac Mass UTM converter provides the user with separate data fields to supply these digits. Otherwise, you'd have to repeat them as you completed every number.

5. UTM Zone Boundaries
The UTM co-ordinate system is not universal like lat-long, it is divided into zones about 500km wide. The full UTM grid references are only unique within a zone. Therefore, to specify where you are on the planet, you need to give the zone number as well as the grid reference.

On the bivouac mountain pages, the zone number is written immediately before the grid reference. It is something like "11 U" for the Rockies, and 10 U for Vancouver area. Below is the FULL grid reference for Mount Robson, including the zone:

11U 355644 5886712
In the above, the "11 U" is the zone. See UTM Zones and the UTM System.

Zone Boundaries Grid lines on 1:50,000 maps near zone boundaries are labelled with two different UTM numbers, one in each zone. That means a given point can have two different coordinates, depending on the zone.

To convert such truncated grid references back to a full UTM, you must use the proper set of leading digits. From a paper map, look in the lower

Eg: Sage Creek (82 G/1) is near the 114 degree boundary. In the lower right corner is two eastings: 718000 and 280000. (In different colors). Obviously, to restore the full grid reference in an area like this, you have to use the leading digit that corresponds to the grid reference that someone gave. So if you have a bunch of grid references like 181-340 to convert, the leading digit of the easting is "7", but if they were numbers like 810-340, the leading digit for the easting is "2".

UTM zone boundaries occur at 114 degrees west, 120 degrees, 126 degrees. The 1:50,000 maps right at the UTM boundaries often quote the easting according to both zones. In other words the zones overlap.

Within a zone In addition to actual UTM zone boundaries mentioned above, the 2 digit grid line numbers obviously repeat every 100km. Again, the leading digits will vary, and you have to make sure you use the correct digit. These maps will have grid line labels like 97, 98, 99, 00, 01, 02. Right at the transition, they will give a full label. What I always do in this case is translate the UTM numbers in two groups: the ones like 98,99, and the ones like 00, 01, 02. Easiest is to look at the map and jot down the proper leading digits. If you don't have a map, pick a nearby peak.

The bivouac mountains are only labelled within a single zone. Best policy if you are an author is to write down at least one full grid reference from the map.

6. Using Paper Maps and the Converter
If you want to use paper maps to get the waypoints, the way you have to do it to get them accurate is to read the grid references off your map, and then use the "Mass UTM Converter" at the bottom of the Authors menu. I think you'll find it most convenient to use "Truncated Grid References.

To teach yourself how to use it, put in the grid references for Parrish mountain, and then convert to WGS84 Lat-long, and check that your answer matches what is on the mountain page.

Here is the info you'll need to work the converter (which I got from the Parrish Mountain page). The full UTM code for Parrish (NAD 27) is

11U 674415-5493923.

The "truncated" digits (which is what you always use, are 744-939. Right? (Check that on your map to make sure you are still with me)

Now go to the converter, and put in the waypoint 744-939. The other info you can get from the above:

Zone 11U
  Leading digit for Easting: 6
  Leading digits for Northing 54.

Make sure you know where I got those from. I wrote a full explanation of all the theory here: PgxPg.asp?PgxId=155